1. Field of the Invention
The present invention relates to a method and apparatus for printing cream solder on a workpiece.
2. Description of the Related Art
In a conventional cream solder printing apparatus and method, a poor printing plate removal property causes cream solder to be left at an opening portion provided in a mask, if a ratio of a depth of the opening portion to a diameter of the opening portion (hereinafter referred to as an aspect ratio of the opening portion) is not smaller than 0.3. For example, in the case where the mask is 0.1 mm thick, if the opening diameter is not greater than 0.3 mm, the cream solder remains there, and even in the case where the mask is 0.4 mm thick, if the opening diameter is 1 mm, the cream solder remains there.
As a method for improving the printing plate removal property, there is a known printing apparatus in which a pin corresponding to the opening portion of the mask is provided above the mask so that the cream solder filled in the opening portion is pushed downwardly (for example, see Japanese Patent Application Laid-open No. Hei 6-238865). In addition, a screen printing method and a screen printing apparatus for improving the printing plate removal property have been proposed in which an internal pressure (air pressure) of an upper sealed portion of the mask is temporarily increased when or before the substrate is separated away from the mask during the printing process, resulting in a good shape printing property (see Japanese Patent Application Laid-open No. Hei 7-9658).
The conventional method for improving the printing plate removal property of the cream solder in which the cream solder, filled in the mask opening portions is pushed out by pins or the like, suffers from a disadvantage that it is difficult to suitably adjust the clearance between the opening portions and the pins in the case where there are dozens of pins or the diameter of the opening portions is small.
FIG. 7A shows the conventional case disclosed in Japanese Patent Application Laid-open No. Hei 7-9658. Reference numeral 9 denotes a mask having opening portions which are filled with cream solder, numeral 10 denotes a squeeze, numeral 14 denotes a gas charge/discharge pipe and numeral 32 denotes a cover. This publication fails to show how the internal pressure is controlled under the state in which the cover 32 is fitted containing the squeezee 10. However, in any case, in this method, since the internal pressure above the mask is not limited to the opening portions filled with the cream solder but to the entire portion of the mask, the mask 9 is deformed downwardly as shown in FIG. 7B. Assuming that the cream solder filled in the opening portions of the mask 9 is all pushed out at the same rate at the opening portions, a height of the tip end of the cream solder extruded from the opening portions of the mask 9 varies depending upon the place corresponding to the deformation of the mask 9.
The cream solder pushed into the opening portions by the squeezee 10 is bonded with a weak adhesive force. In order to normally print the cream solder onto a workpiece, the cream solder has to be transferred under the bonded condition with the adhesive force. When the separation rate of a workpiece 1 and the mask 9 is suitable for the central portion of the mask 9, the separation rate is increased at the peripheral portion of the mask 9 corresponding to the warpage of the mask 9 by the extruding rate of the cream solder. Therefore, the solder is cut causing the extruded cream solder following the cut to be poor. On the other hand, when the separation rate between the workpiece 1 and the mask 9 is suitable for the peripheral portion of the mask 9, since the separation rate at the central portion of the mask 9 is decreased corresponding to the warpage of the mask 9 by the extruding rate of the cream solder, the cream solder is buckled to stick to the bottom surface of the mask 9. The cream solder sticking to the mask 9 causes a fault. Thus, according to this method, it is impossible to applying the cream solder uniformly to the entire surface of the mask 9.
Accordingly, the present invention have made an improvement to a structure in which, a lower surface is provided on a pressure head, and holes corresponding to opening portions of the mask are arranged in the lower surface so that a pressure of gas may be applied only to cream solder filled in the opening portions of the mask. The inventors have found that the cream solder having a high ratio between a height of the cream solder and a diameter of the cream solder (hereinafter referred to as an aspect ratio of the cream solder) may be printed on the workpiece without deforming the mask 9.
In view of the foregoing finding, an object of the present invention is to provide a cream solder printing apparatus and a printing method therefor that may print the cream solder at a predetermined position of the workpiece stably even in case of a mask having a high aspect ratio of the opening portion, a mask having a fine diameter (opening area) of the opening portions and a mask having a wide printing area. Also, since the transferability and the printing plate removal property are in tradeoff relation in the cream solder in some cases, another object of the present invention is to use a cream solder having other properties such as the transferability enhanced by reducing the printing plate removal property.
In order to attain these and other objects, according to the present invention, there is provided a cream solder printing apparatus comprising: a positioning means for positioning a workpiece, a mask means composed of a mask having a plurality of opening portions, a filling means for filling cream solder in the opening portions, a pressurizing means composed of a pressure head having a pressure container and a gas charge/discharge pipe, a position registration means for aligning the workpiece, the mask and the pressure head, and a moving means for peeling the mask away from the workpiece, wherein a lower surface of the pressure head has holes at positions corresponding to the opening positions, such as when the pressure head is aligned with the mask, the holes align with the opening portions.
Also, in the cream solder printing apparatus, it is characterized in that an aspect ratio of the opening portions is in the range of 0.3 to 3.
In order to attain those and other objects of the present invention, according to the present invention, there is provided a cream solder printing method for performing a print with a cream solder by using a mask at a predetermined portion of a workpiece, comprising the following steps of: positioning the workpiece and/or the mask in place, performing registration of opening portions formed in the mask with the predetermined portion of the workpiece, filling the opening portions with the cream solder, performing registration of the opening portion filled with the cream solder with holes formed corresponding to the opening portions at a lower surface of a pressure head, pressurizing the cream solder filled in the opening portions while increasing an internal pressure within the pressure head, peeling the mask away from the workpiece, and moving the workpiece on which the cream solder has been printed to a next process.
Also, in the cream solder printing method, it is characterized in that the internal pressure within the pressure head for extruding the cream solder filled in the opening portions is in the range of 0.002 to 1 kg/cm2.